INOM EXAMENSARBETE TEKNIK, GRUNDNIVÅ, 15 HP , STOCKHOLM SVERIGE 2019 From Bloomery Furnace to Blast Furnace Archeometallurgical Analysis of Medieval Iron Objects From Sigtuna and Lapphyttan, Sverige ANDREAS HELÉN ANDREAS PETTERSSON KTH SKOLAN FÖR INDUSTRIELL TEKNIK OCH MANAGEMENT
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INOM EXAMENSARBETE TEKNIK,GRUNDNIVÅ, 15 HP
, STOCKHOLM SVERIGE 2019
From Bloomery Furnace to Blast FurnaceArcheometallurgical Analysis of Medieval Iron Objects From Sigtuna and Lapphyttan, Sverige
ANDREAS HELÉN
ANDREAS PETTERSSON
KTHSKOLAN FÖR INDUSTRIELL TEKNIK OCH MANAGEMENT
Abstract During the Early Middle Ages, the iron production in Sweden depended on the bloomery
furnace, which up to that point was well established as the only way to produce iron.
Around the Late Middle Ages, the blast furnace was introduced in Sweden. This made it
possible to melt the iron, allowing it to obtain a higher carbon composition and thereby
form new iron-carbon phases.
This study examines the microstructure and hardness of several tools and objects
originating from archaeological excavations of Medieval Sigtuna and Lapphyttan. The aim is
to examine the differences in quality and material properties of iron produced by
respectively blast furnaces and bloomery furnaces. Both methods required post-processing
of the produced iron, i.e. decarburization for blast furnaces and carburization for
bloomeries. These processes were also studied, to better understand why and how the
material properties and qualities of the items may differ. The results show that some of the
studied items must have been produced from blast furnace iron, due to their material
composition and structure. These items showed overall better material quality and
contained less slag. This was concluded because of the increased carbon concentration that
allowed harder and more durable structures such as pearlite to form.
The study also involved an investigation of medieval scissors, also known as shears, made
from carburized bloomery furnace iron. Here, one specific aim was to find out if the
different sections of the shears had different properties, and if so, if these properties
correlated with the functions of the different parts of the shears. Our microstructure
analysis showed that the blade indeed was the hardest part due to intentional carburization
and forming of martensite. The blade is connected to a softer ferritic handle, which in turn is
connected to a ductile bow, also ferritic but with a larger grain size.
Keywords: bloomery furnace, blast furnace, iron production, Middle Ages
Sammanfattning Den svenska järnproduktionen var under medeltiden beroende av blästerugnen som då var
väl etablerad i hela landet. Under denna period introducerades även masugnen i Sverige,
vilket gjorde det möjligt att smälta järn. Den nya tekniken gjorde det möjligt att uppnå en
ökad kolhalt och därmed bilda nya järn/kol-faser.
Den här studien undersöker mikrostrukturer och sammansättningar i medeltida järnföremål
från arkeologiska utgrävningar i Sigtuna och Lapphyttan. Syftet är att undersöka vilka
egenskaper och materialkvaliteter som gick att uppnå i järn som tillverkats med masugn
respektive blästerugn. Båda tillverkningsmetoderna kräver efterbearbetningar som
förbättrar järnets egenskaper. Även dessa efterbearbetningsmetoder studeras, för att kunna
dra slutsatser angående hur det slutligt producerade järnets egenskaper och
materialkvalitet skiljer sig mellan de två produktionsmetoderna. Analyserna visade att järn
som tillverkats i masugn innehöll mindre slagg och generellt var av bättre kvalitet. Detta järn
innehöll även mer kol vilket tillät perlit att bildas. Därmed blev järnet betydlig hårdare.
Slutsatsen är att dessa järnföremål hade övergripande bättre mekaniska egenskaper och
materialkvalitet än de järnföremål som tillverkats med järn från en blästerugn.
I studien undersöks även medeltida fårsaxar, tillverkade av järn från en blästerugn som
sedan genomgått uppkolning. Syftet är att utifrån mikrostrukturen i materialet ta reda på
om de olika sektionerna i en fårsax har olika egenskaper, och om dessa egenskaper i så fall
är anpassade till den aktuella sektionens användning.Analysen visade att bladet var hårdast
på grund av avsiktlig uppkolning och martensitbildning. Därefter följde ett mjukare handtag
med en ferritisk struktur. Den böjda delen av saxen var den mest duktila och fjädrande,
eftersom den uppvisade en rent ferritisk struktur med större kornstorlekar än i handtaget.
3.6 Microstructure and analysis ........................................................................................................ 10
3.7 Vickers test .................................................................................................................................. 10
3.8 SEM ............................................................................................................................................. 11
Table 5. Properties of the investigated objects: excavation background, observed microstructures, Vickers hardness values for different phases, estimated C wt.%, and notations to highlight individual factors that are of interest for each sample.
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5. Discussion The introduction of the blast furnace made it possible for smiths during the middle ages to
achieve a much higher temperature than that of the bloomery furnace. The high
temperatures achieved in the blast furnace now allowed the iron to melt, which significantly
alters the solubility of carbon and the metallic structures. The blast furnace used in
conjunction with a finery forge could now produce forgeable iron in the form of osmunds.
Our hypothesis is that the osmunds had significantly better properties compared to what
was produced in a bloomery furnace in conjunction with carburization. This hypothesis is
here explored by comparing the differences in chemical and structural composition together
with the hardness for several archaeological finds.
A secondary hypothesis is explored regarding the two single bow blade shears that are
believed to have properties that differ in the blade, shaft and bow. To investigate whether
this hypothesis holds up, a metallographic analysis and Vickers hardness test was conducted
for each individual part.
The exact values derived from the Vickers hardness test and estimated carbon composition
can be found in table 5.
5.1 Ingots The ingots that have been analysed show a wide variety of structural difference and carbon
content. Ingot 2 (2078) is made almost entirely out of ferrite which indicates a very low
carbon content below 0.02 C wt.%. The sample contains some form of unwanted inclusions
and is of rather poor quality as well as uneven grain size. It is forgeable due to the soft and
ductile nature of ferrite were the Vickers hardness test show a hardness of 112 HV200,
which can be found in the table 5 with all other Vickers hardness results. Due to the low
carbon content and low quality this ingot is determined to have been produced using a
bloomery furnace. Ingot 6 (40019) most likely falls under this category as well due to the
high amount of slag seen throughout the entire sample. This could also be due to the
unusual composition which is seen in the SEM analysis in chapter 4.3.1. The SEM analysis on
the surface layer for this sample shows very high amounts of phosphorus at around 10 at.%
as well as noteworthy levels of magnesium, sulphur and zirconium between 0.5-1.9 at.%.
The data collected from the Vickers hardness test show that the martensite in this sample is
significantly softer in comparison to the other martensite phases and is most likely due to
the high amount of silicone seen in the second SEM analysis which examines the core. In
Ingot 7 (8636), spherical graphite particles surrounded by ferrite can be observed. This is a
form of ductile cast iron which is less brittle than the typical cast iron and is therefore
forgeable.
Within Ingot 1 (1199), Ingot 3 (2599) and Ingot 4 (3689) high amounts of pearlite can be
observed. This indicates a high carbon content estimated to be around 0.78 C wt.% and
could not have been achieved with the use of a bloomery furnace. Therefore it is concluded
that they were made using a blast furnace and then they have been decarburized using a
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primitive finery forge. Just tapping the iron from the blast furnace without further process
would otherwise yield cast iron which is seen in Ingot 5 (27791). The main difference here is
that the cast iron is not forgeable due to the high carbon composition which can be
observed in the form of graphite scales in figure 28-30.
Judging by appearance alone these samples do not show the typical shape and weight of an
osmund. However, it is most likely that Ingot 1 (1199), Ingot 3 (2599) and Ingot 4 (3689)
have been forged from an osmund judging by their carbon content and included phases.
They might have been discarded due to defects such as cracks or just be a residual piece
from the forging of another tool. When looking at the Vickers hardness for these samples
they present a much higher hardness when compared to the previously named ingots that
originate from a bloomery furnace. The cast iron from Ingot 5 (27791) also show a high
hardness but is on the other hand not forgeable, making it less practical and harder to
process.
5.2 Knives
Ferrite is the most present phase in both knives which is accompanied by pearlite in
localised areas. Slag inclusions can be seen throughout both samples. They can both be
classified as low carbon steel where the pearlite is the result of carburisation to increase the
hardness and wear resistance of the blades. The carburisation differs quite a lot between
the two blades as to where it has occurred and amount of carbon that has been achieved.
Highest amount of carburization can be seen at the edge of Knife 1 (3774) in figure 35,
where the biggest effect is seen at the very edge. The clear pearlite structure is a sign that
the blade has been heated to allow for austenitization during or after the carburization.
Knife 2 (8415) presents carburization in the core which has most likely been achieved by
laminating three pieces together.
The amount of pearlite present in the material is an indicator to how well the item has been
carburized as it is localised to specific areas and greatly affects the hardness. This can be
seen by comparing the very edge of each blade were Knife 1 (3774) contains 68% pearlite
with a hardness of 211 HV200 and Knife 2 (8415) with less than 9% pearlite and a hardness
of 147 HV200. The use of carburization to increase the carbon content and the amount of
inclusions indicates that the iron used for these knives has been produced using a bloomery
furnace.
5.3 Shears
With a quick glance at the microstructure in figures 43-50 it becomes obvious that the
shears are rather different from one another.
Shear 1 (27182) is made entirely with a ferritic structure where the bow shows a much
larger grainsize that makes it more flexible than the rest of the handle. The results from the
SEM analysis in chapter 4.3.3 shows that the ferrite phase contains pure iron. Yet again
carburization can clearly be seen in the blade but in contrast to the knives it is now seen at
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the sides of the blade with a hardness at 209 HV200, surrounding a softer more ductile core
ranging between 120-133 HV200. This should allow the blade to be quite flexible yet able to
retain a sharp edge. The slag lines visible in this area might be the result of folding the iron
between each carburization session to further increase the amount of carburization
achieved.
The second shear called Shear 2 (23276) has similar structure in the handle that is mostly
ferrite, apart from some pearlite seen in the top left corner of figure 41. The blade differs
from the other shear where the entire blade now contains nothing but martensite. The
martensite structure has been acquired by rapidly cooling the blade from the austenite
phase which can be done even if the carbon content is low. This makes this blade much
harder than the previous shear which is highlighted by the Vickers test at 894 HV200. The
carbon content of martensite cannot be accurately estimated from the structural analysis
but can be considered as low in carbon due to a low carbon composition in the shaft and by
comparison to the other shear.
The hypothesis put forth was that the properties of different sections of the shears would
vary depending on what properties would be beneficial. By comparing the grainsize found in
the blade and bow of shear 1, which can be observed in figure 43-44, it is clearly seen that
the grain size in the blade is drastically smaller. This difference in grainsize together with the
carburization that has been observer, shows that the properties indeed was altered to
benefit each section and its intended use.
Even though the two shears differ from a structural standpoint it´s safe to assume that both
have been forged from iron produced in a bloomery furnace. The two shears differ in size,
as Shear 2 (23276) is around 20cm in length which makes it about twice the length of Shear
1 (27182). It´s rather interesting that the bigger shear has a much harder blade which might
imply that the intended purpose for each shear differ from each other. To put this into
context the smaller shear seems to be most suited for fine work such as cutting hair or
different fabrics where the strain on the tool is limited. The bigger shear is comparably more
suited for work that includes tougher materials such as cutting twigs or extended repetitive
use that otherwise would dull the edge of a softer blade.
5.4 Vickers hardness test The data from the Vickers hardness test in figure 51 show some variations that should be
highlighted. The exact values obtained from the Vickers hardness test can be found in table
5. Ingot 4 (3689) and Ingot 1 (1199) both contain pearlite which is much harder than what is
observed in the other samples. This is believed to be due to the high carbon composition in
both samples as well as the presence of martensite. The martensite might have been just
below the tested martensite and thereby affected the result. The grey cast iron in Ingot 5
(27791) also show a rather significant difference even though they all share the same phase.
They key here is the amount of graphite present in the area that has been tested, the
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Vickers hardness of 535 HV500 has been found at the edge of the sample where the amount
of graphite was highest.
5.5 Ethical aspects
It is a bit of a moral dilemma we are facing when it comes to destroying these centuries old
finds and objects. On one hand destroying them will make it possible for us to research and
understand how they were made and how they were used. It is also a piece of history that
tells us of our past and preserving it will enable the past to be remembered. Even though,
when visiting the vast collections of items from this period, we feel it is morally justified to
destroy, examine and experiment on just a small part of it. Because, just looking at an object
does not give the whole story, and instead of collecting dust these objects may help us learn
more about our history which we feel is worth it.
Due to the nature of this project being arkeological, the environmental effects of our studies
are nominal.
However, the environmental effects of the invention of the blast furnace and the increase of
iron production during the middle ages as well as the start of mining had a large
environmental effect. Because of the difference in scale between the blast furnace and the
bloomery, and because the blast furnace could run nonstop, the demand for wood coal
must have increased drastically. Leaving the areas around these sites to become deforested
if production was upheld for a long period of time. The slag, which sometimes contained
toxic elements, was left as a biproduct and could contaminate the water and surrounding
environment if not handled properly.
5.6 Sources of error
When studying objects that are this old there are going to be a lot of areas where
uncertainties and insecurities may occur. Because the samples have been under the ground
for hundreds of years the objects studied have been corroded, some heavily, to the point of
having no solid core left in some places. This makes it impossible to determine the
microstructure or if there have been exposed to surface hardening, such as carburization, as
that would have corroded away. It can also mean that some of the results may be faulty.
Another aspect is measuring errors. When doing the Vickers Hardness test the diagonals
made to determine the size of the indentation are made by hand and will have a small
margin of error, leading to a slightly different result.
The locations of the finds also bring uncertainties. Especially the finds from Sigtuna, as that
place was, during the time these objects were made, a larger town with lots of trading of
different items and materials. This makes it very hard to determine where the objects could
have originated from, if the iron ore came from the same place or if the objects had been
manufactured elsewhere and then bought or traded. Lapphyttan does not have the same
level of uncertainties because it was a production facility which made steel.
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6. Conclusion
From the samples that have been analysed it can be concluded that the ones thought to
have been produced with the use of a blast furnace show very good properties, especially
increasing the hardness of the pearlite. The amount of pearlite within these samples are
also much higher and more widespread which increases the strength throughout the entire
object significantly. As it was theorized these samples show a much better consistency and
better properties compared to the objects made using a bloomery furnace.
With that said, the objects made using a bloomery furnace should not be overlooked as they
can still be made into tools with good quality and properties that are modified to better suit
their intended use. This is clearly proven by the single bow blade shears where the
properties can be directly corelated to the intended purpose. The theory that was put forth
was that the shear vas constructed with a ductile bow, a slightly harder shaft and a much
harder blade which was concluded to be in line with this conclusion.
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7. Further investigation There are several topics in this report that can further be analysed and researched. The first
being the continuation of this project by examining more objects to better understand the
variation of properties and quality. A more in-depth SEM analysis could also be made to
better understand the irregularities in composition that is quite common in medieval iron
objects. This could also be used to determine which ore that has been used and how this
affected the properties and quality. Another thing is to examine a piece of confirmed
osmund iron and compare it to the pieces that we believe originate from an osmund. This
should be done to see if our assumption is correct and if the iron has been altered by any
later forging process.
There is also the case to further investigate the first production techniques in creating iron
from a blast furnace and more specifically the finery forge as very little is known about this
process. An experimental physical recreation of the decarburization process will be
conducted at Nya Lapphyttan in the summer of 2019.
All the tools examined in this report comes from iron made in a finery forge. It would
therefore be useful to examine tools that have been made using iron originating from a
blast furnace to see how the properties and quality vary in the final product.
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8. Acknowledgement We would like to thank everyone who helped us along the way for making this project
possible. First of all, we would like to thank Sigtuna Museum for lending us the items and
letting us examine them. To Gert Magnusson, Docent in archaeology, we want to extend a
big thanks for letting us borrow and examine samples and for providing us with valuable
insight and assistance. We would also like to express our gratitude to our mentors in this
project, Sebastian Wärmländer at the Biochemistry and Biophysics Department at
Stockholm University and Anders Eliasson at the Department of Materials Science at the
Royal Institute of Technology. We would like to thank Wenli Long at the Department of
Materials Science at the Royal Institute of Technology for assisting us with the preparation
an analysis of our samples. Thanks to Eva Hjärtner-Holdars, Docent in archaeology, and Lars
Bentell, Licentiate in metallurgical chemistry, for taking the time to discuss our project and
provide valuable insight.
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9. References 1. C. Karlsson, Förlorat Järn – det Medeltida Jordbrukets Behov och Förbrukning av Järn och Stål,
Halmstad, 2015, 67-69.
2. Ortshistoria "Sigtunas historia" http://ortshistoria.se/stad/sigtuna/historia [28 may 2019]
3. Norbergs Kommun "Lapphyttan och bergsmansbyn Olsbenning"